CHAPTER 13

Messages

SENDING

We have concluded, then, that there may well be over 500,000 civilizations in the Galaxy, but that the only way any of them are likely to emerge from their planetary systems is by interstellar probes or in the form of free-worlds.

There is nothing compelling about either emergence. The vast majority of civilizations, conceivably all of them, may simply remain in their own planetary systems. Any interstellar probes that are sent out may be devices not designed to land on habitable planets but to confine themselves to observations and reports from space. Any free-worlds that may come our way might be more interested in material and energy with which to maintain themselves than in involvement with a sedentary civilization.

In this way, we can rationalize the apparent paradox that while the Galaxy may be rich in civilizations we remain unaware of them.

But what ought we to do in that case?

The simplest answer and the one that involves the least trouble is to do nothing at all. If extraterrestrial civilizations can’t or won’t reach us, we could just go about our own business. Certainly we have enough troubles of our own to occupy us.

The second possibility is to send out some sort of message in order to make contact. Even if an extraterrestrial civilization can’t reach us, or we them, we can perhaps establish communication across space; even if it is only the message: “We are here. Are you there?”

This is such a normal impulse that back in the nineteenth century, when people were still speculating concerning life on other worlds in the Solar system and almost taking it for granted that there would be civilizations even on the Moon, there were suggestions for methods of communication.

The German mathematician Karl Friedrich Gauss (1777–1855) once suggested that lanes of forest be planted on the steppes of central Asia in the form of a gigantic right triangle with squares on each side. Within the triangle and squares, grain would be planted to darken the shapes with a uniform color. A civilization on the Moon or Mars, for instance, closely studying the surface of the Earth, might see this clear display of the Pythagorean theorem and would conclude at once that there was intelligence on Earth.

The Austrian astronomer Joseph Johann von Littrow (1781–1840) suggested instead that canals be dug, and that kerosene arranged in mathematical forms be floated on the water and set on fire at night. Again, mathematical symbols would be seen from other worlds.

The French inventor Charles Cros (1842–1888) suggested something more flexible—a vast mirror that could be used to reflect light toward Mars. It could then be so manipulated as to send the equivalent of Morse code and actual messages could, in this way, be sent (though they might not necessarily be interpreted, of course).

Interest in establishing communication with extraterrestrial civilizations mounted to the point where, in 1900, a prize of 100,000 francs was offered in Paris to the first person to carry through this task successfully. Communication with Mars was excluded, however. That was thought to be too easy a feat to be worth the money.

All such nineteenth-century suggestions are useless, of course, since there are no intelligent beings on the Moon, Venus, or Mars, and it is doubtful whether the unsophisticated techniques suggested could reach farther (if, indeed, that far).

Besides, in the twentieth century we have, ironically enough, sent out even more spectacular messages with no special effort on our part.

The invention of the electric light and the gradually increasing illumination of our cities and highways has steadily intensified the glitter of Earth’s surface at night, at least over the land areas that are industrialized and urbanized. Astronomers on Mars, puzzling over the light emerging in steadily increasing intensity from Earth’s dark side would be sure to come to the conclusion that a civilization existed on Earth—if there were astronomers on Mars.

The nineteenth-century suggestions made use of light, since that was the most easily manipulable radiation known to cross the vacuum of space at that time. Around the turn of the century, however, radio waves were discovered (like light waves, but a million times longer) and put to use. By 1900, the Yugoslavian-American inventor Nikola Tesla (1856–1943) was already suggesting that radio waves be used to send messages to other worlds.

No deliberate attempt of the kind was made, but it didn’t have to be. With the passing decades radio waves were generated by human beings with ever increasing intensity. Those that could penetrate the upper layers of Earth’s atmosphere did so, and as a result there is a sphere of radio-wave radiation swelling out from Earth in every direction.

Again, astronomers on Mars, if they were aware of this radiation and if they noted that it was growing steadily stronger, would be forced to come to the conclusion that there was a civilization on Earth.

By the second half of the twentieth century, however, it was quite clear that extraterrestrial civilizations did not exist in the Solar system and that if we were to send messages it would have to be to the stars.

This introduced formidable complications. In the Solar system, we at least know where we might aim our messages—at Mars, at Venus, and so on. There is, on the other hand, no way of knowing which star it would be best to aim at.

Furthermore, radiation aimed at the stars would have to be very energetic if it were to maintain sufficient intensity, in view of inevitable dispersion over the light-years, for it to be picked up at even the distance of the nearest stars.

We are, as I have already said, sending out radio-wave radiation to the stars quite involuntarily. The radio waves that have leaked through the upper layers of our atmosphere have expanded now into a vast ball dozens of light-years in diameter. The outer fringes have passed by many stars already, and although the intensity is excessively minute, it could conceivably be picked up.

However, signals so excessively weak might not seem to the distant astronomers to be incontrovertible proof of a civilization existing somewhere in the neighborhood of our Sun. Even if the astronomers came to the conclusion the civilization existed, the complicated mix of signals would be impossible to sort out and make sense of.

A deliberately emitted beam of radiation could be designed to contain a great deal of information and could be made strong enough to remove all doubt even if its content could not be interpreted.

The trouble is that we do not at the moment want to dispose of the energy to spray messages out into space, especially since we aren’t sure of any specific target, and cannot honestly have much hope of an answer until, at best, many years have passed.

Is there something we can do that will cost less in terms of energy?

We might send a material message, something we can cast arbitrarily into space at little or no cost. To be sure, a material message would be harder to aim than a beam of radiation, and the material message might take many thousands of times longer to get to any specific destination, but at least it would be well within our present capacities.

And the fact is that we have sent a message.

On March 3, 1972, the Jupiter probe, Pioneer 10, was launched. It passed by Jupiter in December 1973, making its closest approach on December 3, and very successfully sent back photographs and other data that enormously increased our knowledge of that giant planet.

If that were all—if, after having passed Jupiter, Pioneer 10 had vanished, or exploded, or simply gone dead—it would have proved worthy of the time, effort, and money expended on it. Anything it could do beyond the Jupiter mission was, in a way, an added bonus. Adding a message to it, therefore, would cost virtually nothing.

Pioneer 10 does carry a message, one that was added at the last minute as a matter of sheer bravado.

The message is a gold-anodized aluminum plate, 6 inches by 9 inches, which is attached to the antenna support struts of Pioneer 10.

Etched onto the plaque is informational matter that was decided on by the American astronomers Carl Sagan and Frank Donald Drake. Most of the information would be completely over the heads of all but a very few human beings. It involves details concerning the hydrogen atom, and that information is expressed in binary numbers. It locates the Earth relative to nearby pulsars, giving the periods of the pulsars in binary numbers. Since pulsars are in a particular place only at particular times, and since their rate of rotation slows so that they will have the given rate for only a period of time, this information tells exactly where the Earth has been relative to the rest of the Galaxy at a particular time in cosmic history.

There is also a small diagram of the planets of the Solar system and an indication of Pioneer 10 itself and the path it took in going through the Solar system.

The most noticeable item on the plaque, though, is a diagrammatic representation of Pioneer 10 and in front of it, to scale, an unclothed man and woman (drawn by Linda Salzman Sagan, Carl’s wife). The man’s arm is lifted in what (it is hoped) will be interpreted as a gesture of peace.

If an intelligent species should happen to pick up the message, will it be understood? Since it is almost as certain as anything can be that it will be picked up only by some species in a spaceship or a free-world, we can suppose that species will have developed a technology that will possess advanced scientific concepts. They should, therefore, certainly grasp the meaning of the purely scientific symbols. Sagan points out, however, that it is the drawing of the human beings that may puzzle them, since the pictures may be like no form of life they have ever encountered. They may not even interpret the markings as representing a life form.

They will also have Pioneer 10 itself to study and, in some ways, that may tell them more about Earth and its inhabitants than the plaque will.

But where is Pioneer 10 taking the plaque? Pioneer 10, as it skittered around Jupiter, gained energy from Jupiter’s vast gravitational field, and by 1984 it will coast past Pluto’s boundary at a speed of 11 kilometers (7 miles) per second. That will be enough to carry it indefinitely away from the Sun, wandering on for billions of years unless it strikes an object large enough to destroy it.

It will take Pioneer 10 about 80,000 years to recede from us to a distance equal to that of Alpha Centauri. It will not be anywhere near Alpha Centauri at that time, however, for it is not going in that direction.

Pioneer 10 was not aimed with any star in mind, after all. It was aimed at Jupiter in such a way as to give us maximum information about that planet, and whatever direction it took up thereafter, on leaving the Solar system—that was it.

As it happens, Pioneer 10 will be following a path that will not come close enough to enter the planetary system of any star we can see for at least 10 billion years. Of course it may through sheer accident skim by a free-world some time in its long journey. The chances of even that must surely be exceedingly small, however, and no one seriously expects that Pioneer 10 will come within the purview of any intelligent species at any time in its long journey.

In that case, why should we have bothered?

In the first place, it was a very small bother. And in the second place, it just might be picked up at some time, and even if those who pick it up are much too far away from us to do anything about it, or if it is picked up at a time long after humanity is extinct, we would nevertheless have made some mark on the Universe.

We would have left behind evidence that once there was an intelligent species on our small world that could manage to put together enough expertise to hurl an object out of our Solar system. There is such a thing as pride!

Finally, we can multiply our chances by sending out more than one message. An identical plaque was placed on Pioneer 11, which will eventually leave the Solar system on a track different from that of Pioneer 10.

And in 1977, probes were launched on which were included numerous photographs showing widely mixed aspects of life on Earth, together with a recording containing enormously varied sounds produced on Earth.

RECEIVING

Obviously, it will be some time before we are in a position to send out messages that are more than passive cartoons, aimed virtually at random.

Furthermore, there is some opposition to the thought of sending out messages at all. The nub of that opposition rests with the question: “Why attract attention?”

Suppose we do announce our presence. Are we not simply inviting civilizations advanced beyond ours, which have hitherto not been aware of our presence, to make for us at full speed and to arrive with the intention of taking over our world, of reducing us to slavery, or of wiping us out?

The chances seem to me to be strongly against that. I have explained earlier in the book why I consider it very likely that civilizations that have advanced beyond our own level of technology will be peaceful. Even if not peaceful, civilizations are very likely confined to their own planetary systems. In the very unlikely case that a civilization is warlike and is also roaming freely through space, it has probably examined all stars and is aware of our presence. Finally, even if it has unaccountably missed us, we have already given ourselves away by our radio broadcasts.

For all these reasons, it makes no difference whether we signal or not, and yet it is hard to answer the unreasoning fears that assume the very worst combination of possibilities. Suppose there are civilizations out there as vicious and warlike as we ourselves are at our worst, who can move through space freely, who are looking for new prey, and who have until now been unaware of us. Shouldn’t we lie low and keep absolutely quiet?

Accepting that argument, should we not, for our own safety, find out as much as we can about these hypothetical monsters even while we are lying low? Shouldn’t we want to know where the danger is, how bad it might be, how best we might defend ourselves, or (if that is impossible) how best we might more effectively hide?

In other words, abandoning any attempt to send messages (at which we are ineffective, in any case) ought we not to make every attempt to receive messages? If we do receive a message and decipher it and decide we don’t like what we hear, there is, after all, no reason why we would have to answer it.

Would we, however, know we had come across a signal if we detected it? What ought we to look for?

We might take the optimistic attitude that though we can’t predict what the signals would be, we would recognize them if they were there. The detection of what seemed to be a network of Martian canals was a complete surprise, but was quickly taken as an indication of a high civilization.

We know now, though, that if life signals are obtained from anywhere it will have to be from the planetary systems of other stars (or possibly from automatic probes or free-worlds in interstellar space). The likelihood is that any signals we do get will come from many light-years away, and the question is whether it is reasonable to suppose that signals energetic enough to make themselves felt across such distances could be sent out.

It might be that we should not judge all civilizations by our own. What seems a high energy level to ourselves might not seem high at all to more advanced civilizations. In 1964, the Soviet astronomer N. S. Kardashev suggested that civilizations might exist at three levels. Level I is Earthlike and can dispose of energy intensities of the kind available through the burning of fossil fuels. Level II could tap the entire energy of its star, thus disposing of energy intensities 100 trillion times that of Level I. Level III could tap the entire energy of the galaxy of which it is a part, thus disposing of energy intensities 100 billion times that of Level II.

A signal from a Level-II civilization could easily have enough energy content to be detectable from any part of the galaxy of which it is part. A signal from a Level-Ill civilization could easily have enough energy content to be detectable anywhere in the Universe.

We might dismiss this at once by saying that we detect no signals anywhere but, in the first place, we are not really listening. In the second place, even if the signals forced themselves upon our consciousness, would we recognize them for what they are?

In 1963, for instance, the Dutch-American astronomer Maarten Schmidt (1929–) discovered quasars, extraordinarily bright and distant objects that show irregular variations in brightness. In 1968, the British astronomer Anthony Hewish (1924–) announced the discovery of pulsars, which send out regular pulses of radiation at very short but very slowly lengthening intervals. Beginning in 1971, certain intense x-ray streams that varied irregularly in intensity were ascribed to black holes.

Could it be that these objects represent the signal beacons of Level-II or Level-III civilizations? To be sure, the variations in intensity seem to be quite irregular in the case of quasars and black holes, and quite regular in the case of pulsars, and in either case don’t seem to have the kind of information that would be of intelligent origin—but may that be merely the result of our own inadequate understanding?

Perhaps! From the conservative position of this book, however, it is an extremely unlikely perhaps. We can only say that thus far there is no large-scale phenomenon in the Universe, involving the kind of energy output characteristic in intensity of stars or galaxies, where there is any evidence whatever of intelligent information content. Until such evidence arrives, we must delay a decision.

Of course, a signal might not be a deliberate beacon but the entirely involuntary accompaniment of a civilization’s activities. We are illuminating our cities and highways only for the convenience and safety of human beings, but it turns out to be a signal to any extraterrestrial civilizations that are close enough and attentive enough to note it.

If the Martian canals really existed, they would do so only to supply the Martian civilization with badly needed water for irrigation—but their existence would have signaled us.

In the same way, a more advanced civilization may do something sufficiently enormous to make itself felt at stellar distances.

Freeman J. Dyson suggested that if human beings began to exploit and explore space, they might wish to expand their numbers to the utmost that can be sustained by the Sun’s energy. At the present moment, the Earth stops only a tiny fraction of sunlight, and almost all the solar radiational energy slips past the cool bodies of the Solar system to streak into and through interstellar space. Human beings might therefore eventually break up the various outer bodies of the Solar system to make up a group of free-worlds that will be placed in a spherical shell about the Sun at the distance of the inner edge of the asteroid belt.

All the Sun’s energy would be absorbed and utilized by one or another of the free-worlds. The energy would, of course, be reradiated into space from the dark side of each of the free-worlds, but only as infrared radiation. Viewed from another star, then, the Sun’s radiation would seem to change its character from one in which a major portion was emitted as visible light to one in which almost all was emitted as infrared. The changeover would take perhaps a couple of centuries, the barest instant of astronomical time.

If, then, from our own Earth we should see some other star, which has been shining steadily as far as our records tell us, suddenly begin to lose brightness and after a while blink out, we can be reasonably sure we have seen intelligence at work.

Well, perhaps—but we haven’t seen anything of the sort as yet.

We must come to the conclusion, then, that (1) we are hopelessly inept at detecting signals and might as well not bother; or that (2) no signals are being sent out and that we might as well not bother; or that (3) signals are being sent out but at much less than heroic energy content, and as a result of much less than heroic civilizational activity, and that in order to detect them we will have to make a considerable effort.

Clearly, we cannot accept the first or second conclusions until we have made an honest attempt at the third.

Then let us consider signals of low-energy content (but high-energy enough to detect) and see what they might be like.

They would have to consist of some phenomenon that could cross vast reaches of space, and these can be divided into three classes: (1) large objects such as plaques, probes, and free-worlds; (2) subatomic particles with mass; (3) subatomic particles without mass.

The large objects we can eliminate at once. They move slowly and are extremely inefficient as carriers of information.

The subatomic particles with mass can be divided into two subclasses, those without electric charge and those with electric charge. Subatomic particles with mass but without electric charge generally move slowly and can be eliminated as impractical for that reason.

Subatomic particles with both mass and electric charge can move quickly because they are accelerated by the electromagnetic fields associated with stars and with galaxies as a whole. Therefore, in crossing interstellar and intergalactic spaces, they achieve very nearly the speed of light and, in consequence, enormous energies.

Such subatomic particles do indeed occur everywhere and they are constantly and eternally bombarding the Earth. We call them cosmic rays.

The difficulty here, though, is that the mere fact that these particles are accelerated by electromagnetic fields means that they experience an attraction or a repulsion and that, in either case, their paths curve. As the particles gain increasing energy, their paths curve more and more slightly, but over vast distances even the slightest curve becomes important. What’s more, a beam of particles is gradually dispersed, since those with more energy are curved less than those with less energy.

The cosmic-ray particles bombard us from all sides, but because of their past experiences with electromagnetic fields, there is no way of telling from the direction of their arrival where they came from. Nor can we tell whether a particular group that arrives together left together. For a signal to be of any use, it has to come in a straight line and be neither dispersed nor distorted, and that eliminates all subatomic particles with mass.

We are now left only with the subatomic particles without mass, and there are only three known general classes of such particles:^* neutrinos, gravitons, and photons.

Being massless, all these particles travel at the speed of light and there can be no faster messengers. That is one point in their favor.

Moreover, no massless particle carries an electric charge, so none is affected by electromagnetic fields. They are affected by gravitational fields, but detectably so only in regions where such fields are very intense. Even there, beams of massless particles would bend in unison and would not be dispersed. Since the intensity of the gravitational field in space is negligible almost everywhere, all massless particles reach us in essentially a straight line and essentially un-dispersed and undistorted, even though their origins are billions of light-years away. That is a second point in their favor.

In the case of neutrinos, however, reception is extremely difficult, since neutrinos scarcely interact with matter at all. A stream of neutrinos could pass through many light-years of solid lead without more than a small fraction of them having been absorbed.

To be sure, a very small fraction can be absorbed even in relatively small samples of matter, and so many neutrinos can very easily be produced that such a very small fraction might suffice to carry a message.

However, the type of nuclear reactions that go on in the interior of stars produces neutrinos. In a Sunlike star, vast numbers of neutrinos are produced in this fashion.^* A civilization is not likely to produce more than an insignificant fraction of the neutrinos their own star will be producing, so that there will be the danger that whatever message the civilization sends out will be swamped by the much greater volume of neutrinos the star is emitting. (It is a general rule, perhaps, that the medium you use for your message should be easily distinguished from the background. You don’t whisper a message across a room in a boiler factory.)

There is a possible way out of this. While the fusion reactions involving hydrogen nuclei at the center of the stars produce neutrinos, the fission reactions involving the breakup of massive nuclei such as those of uranium and thorium produce related particles called antineutrinos.

Antineutrinos are also massless and chargeless but are, so to speak, mirror images of neutrinos. When absorbed by matter, antineutrinos produce different results than neutrinos do, and if a civilization is careful to allow a stream of antineutrinos to be the message carrier, it could be read even in the presence of a vast flood of neutrinos.

Nevertheless, the difficulty of intercepting such particles is such that no civilization would use this method if something better were available.

Gravitons, which are the particles of the gravitational field, are certainly not better. Gravitons carry so minute a quantity of energy that they are even more difficult to detect than neutrinos. What’s more, they are far more difficult to produce than neutrinos. To produce even barely detectable gravitational radiation, using the technology currently at our disposal, huge masses must be made to accelerate—through rotation, revolution, pulsation, collapse, and so on—in some pattern that will serve as a code. We can fantasize a civilization so advanced that it can make a giant star pulse in Morse code, but even that advanced a civilization wouldn’t bother if something simpler were available.

That leaves the last category of communications systems—photons.

PHOTONS

All electromagnetic radiation is made up of photons, and these come in a wide variety of energies,^* from the extremely energetic photons of the shortest-wave gamma rays to the extremely unenergetic longest-wave radio waves. If we consider any band of radiation in which energy doubles as we pass from one end of the band to the other (or the wavelength doubles in the other direction) then that is one octave. There are scores of octaves making up the full stretch of electromagnetic radiation, and visible light makes up a single octave somewhere in the middle.

All objects that are not at absolute zero in temperature radiate photons over a wide range of energies. There are relatively few at either end of the range, and a peak somewhere in the middle. The peak represents photons of a certain energy, and as the temperature rises, the peak is located at higher and higher energies.

For very frigid objects near absolute zero, the peak radiation is far in the radio-wave region. For objects at room temperature, like ourselves, for instance, the peak is in the long-wave infrared. For cool stars, it is in the short-wave infrared, though enough photons of visible light are radiated to give the stars a red color. For Sunlike stars, the peak is in the visible-light region. For very hot stars, it is in the ultraviolet, although enough photons of visible light are produced to give the star a blue-white appearance.

Most of the range of electromagnetic radiation cannot penetrate our atmosphere, but visible light can, and most organisms have evolved sense organs that can respond to these photons. In short, we can see.

On Earth, at least, we have the aid of our other senses, but for any object beyond our atmosphere, the only information we have ever received (until very recently) is through the visible-light photons that have reached us from those objects.

It is natural, therefore, that we would think of signals from outer space in terms of visible light. We see the Martian “canals” and extraterrestrials watching Earth would see any markings we deliberately drew on the planetary surface, or the lights of our nighttime illumination.

Signaling by light represents a vast advance over signaling by neutrinos or gravitons. Light is easily produced and easily received. We can imagine some civilization setting up an exceedingly intense beam of light, and flicking it on and off in some way that would make it instantly recognizable as the product of intelligence. For instance, if we represent each flick as *, we might receive, over and over again, **—***—*****—*******—***********—*************—*****************— We would recognize that at once as the first members of the series of prime numbers and could not doubt that we were dealing with a signal of intelligent origin.

There are difficulties, though. A light beam intense enough to be seen at interstellar distances would require vast energies, and even then the light beam would be completely drowned out by the light of the star that the planet circles.

A Level-II civilization might conceivably know of ways to make a star bright and dim in such a way as to make a signal of undoubted intelligent origin, and a Level-III civilization might make a whole group of stars do so. This, however, is pure speculation. Nothing like it has ever been observed and it would certainly be unnecessary to make use of so heroic a signaling device if we can find something simpler.

For instance, what if the signal beam were a kind of light that was not produced in nature? This suggestion might have seemed silly prior to 1960, but in that year the laser was developed by the American physicist Theodore Harold Maiman (1927–), and within a year it was suggested as a possible carrier for interstellar messages.

All light produced in ordinary fashion is “incoherent.” It comes in a wide band of photon energies, and the different photons are generally heading different ways. A beam of such light quickly spreads out no matter how we try to focus it; and to keep it intense enough to be detectable at interstellar distances requires almost stellar energies.

In a laser, though, certain atoms are lifted to a high energy level and are allowed to lose this energy under conditions that produce “coherent” light—light that is composed of photons that are all of equal energies and are all moving in the same direction. A laser beam scarcely spreads out at all, so that for a given energy it can remain intense enough to be detected at far greater distances than a beam of ordinary light. What’s more, a beam of laser light can be easily identified spectroscopically, and merely through its existence is satisfactory indication of intelligent origin.

With laser light we come closer to a practical signaling device than anything yet mentioned, but even a laser signal originating from some planet would, at great distances, be drowned out by the general light of the star the planet circles.

One possibility that has been suggested is this—

The spectra of Suntype stars have numerous dark lines representing missing photons—photons that have been preferentially absorbed by specific atoms in the stars’ atmospheres. Suppose a planetary civilization sends out a strong laser beam at the precise energy level of one of the more prominent dark lines of the star’s spectrum. That would brighten that dark line.

If we studied the spectrum of a star and discovered that it was missing one of the dark lines characteristic of a certain group of atoms in the star’s atmosphere, but that other dark lines also characteristic of that group were present, we would have to conclude that the missing energy level had been supplied by artificial means. That would mean the presence of a civilization.

Nothing like that has been observed—but before feeling depressed over that, let us see if perchance there are still simpler ways of signaling. After all, no civilization would be expected to use the harder method when a simpler is available.

MICROWAVES

Early in the nineteenth century, electromagnetic radiation outside the range of visible light was first discovered. In 1800, William Herschel discovered the infrared range of sunlight by the manner in which a thermometer was affected beyond the red limit of the range of visible light. In 1801, the German physicist Johann Wilhelm Ritter (1776–1810) discovered the ultraviolet range of sunlight by the manner in which chemical reactions were brought about beyond the violet limit of the range of visible light.

These discoveries did not affect astronomy very much, however. Most of the range of ultraviolet and infrared could not penetrate the atmosphere, so that little of it reached us from the Sun and the stars.

Beginning in 1864, Maxwell (who had worked out the kinetic theory of gases) developed the theory of electromagnetism. This first identified light as an electromagnetic radiation and predicted the existence of many octaves of such radiation on either side of the visible light range.

In 1888, the German physicist Heinrich Rudolf Hertz (1857–1894) detected lightlike radiation with wavelengths a million times longer than light and with energy levels that were, therefore, only a millionth as high. The new radiation came to be spoken of as radio waves.

Radio waves, because of their low energy content, turned out to be easy to produce, and despite their low energy content, easy to receive. Radio waves could penetrate all sorts of material objects as light could not. Radio waves could bounce off layers of charged particles in the upper atmosphere as light could not, so that radio waves could, in effect, follow the curve of Earth’s surface. Radio waves could easily be produced in coherent fashion, so that a tight beam could go long distances, and could easily be modified to carry messages.

For all these reasons radio waves were clearly ideal for longrange communication, and that, too, without the wires that telegraphs and cables required. The first to make practical use of radio waves in this way was the Italian electrical engineer Guglielmo Marconi (1874–1937). In 1901, he sent a radio-wave signal across the Atlantic Ocean, a feat generally recognized as the invention of radio.

From that day on, with further improvements and refinements, radio became a more and more important means of communication. It was clear to many people that any technological civilization would surely make use of radio communication in preference to anything else.

Therefore, when the planet Mars made a closer than usual approach to Earth in 1924, there was some attempt to listen for radio signals from the presumed civilization that had built its canals. Nothing was detected.

In a way that was not surprising. The layers of charged atoms in the upper atmosphere that reflected Earth-made radio waves and kept them in the neighborhood of the surface instead of allowing them to pass outward into space would also serve to reflect spacemade radio waves and keep them away from Earth’s surface.

In 1931, however, the American radio engineer Karl Guthe Jansky (1905–1950), working for Bell Telephone Laboratories, detected an odd signal when he was trying to determine the source of static that interfered with the developing technique of radio telephony. It turned out that the signal was coming from the sky. That was the first indication that there was a wide band of short-wave radio waves, called microwaves, that could easily penetrate Earth’s atmosphere. There were two types of electromagnetic radiations that we could get from the sky: a narrow band of visible light and a broad band of microwaves.

By December 1932, it was demonstrated that Jansky had detected radio waves from the Galactic center, and that made front-page headlines in the New York Times. Some astronomers, such as Jesse Leonard Greenstein (1909–) and Fred Lawrence Whipple (1906–), at once appreciated the potentialities of the discovery, but there was little that could be done about it. There were no decent instruments for detecting such radiation. One American radio engineer, Grote Reber (1911–), did take it seriously, however. He built a device to detect radio waves from the sky (a “radio telescope”) and from his back yard, beginning in 1938, studied as much of the sky as he could reach in order to measure the intensity of radio-wave reception from different areas.

During World War II, the development of radar changed everything. Radar made use of microwaves so that microwave technology advanced rapidly, and after the war, radio astronomy quickly became a giant, revolutionizing the science as it had been revolutionized by Galileo’s optical telescope 3½ centuries before.

In just a few decades, radio telescopes have been built that can detect microwaves far more delicately than light can be detected. Sources of microwave radiation could be detected at distances too great for us to make out light radiation of anything like equivalent energy. In fact, we can right now detect microwaves from any star in the Galaxy, even though those microwaves are sent out with no more energy than we ourselves could dispose of.

Then, too, the sources of microwaves can be located with great precision, and the varieties of microwaves can be differentiated with great ease. Every molecule emits or absorbs its own specific wavelength, so that the chemical constitution of interstellar gas clouds can be determined with great precision. Microwaves are not blanked out by background radiation. In most parts of the sky, microwaves are not radiated with the intensity of light, and even where microwaves are plentiful, it would be easy for a civilization to send out a specific wavelength that would be far stronger than the natural background for that wavelength.

It amounts to this: If any civilization is trying to send out messages, it would surely come to the conclusion that microwaves are a better, cheaper, and more natural medium for those messages than light—or, in fact, than anything.

We finally have what looks like the answer. To send, or receive, messages across the interstellar gulfs, we must make use of microwaves.

But at what energy level, or wavelength, ought we to expect the message to come? Receivers can be tuned to receive some specific wavelength, and if the message is being sent at another wavelength, it will be missed. On the other hand, to try to tune in all possible wavelengths would enormously increase the difficulty and expense of listening. But can we read the extraterrestrial mind and guess the wavelength it would choose to use?

During World War II, the Dutch astronomer Hendrick Chistoffell Van de Hulst (1918–), unable to make observations under the Nazi occupation, did some pen-and-paper calculations that showed that cold hydrogen atoms would sometimes undergo a change in configuration that would result in the emission of a microwave photon that was 21 centimeters (8.3 inches) in wavelength.

The individual hydrogen atom undergoes the change only very rarely but, considering all the hydrogen atoms in space, great numbers are undergoing the change at every moment, so that if Van de Hulst’s calculations were correct, the microwaves produced by hydrogen atoms should be detectable. In 1951, the American physicist Edward Mills Purcell (1912–) did detect them.

The hydrogen atom is predominant in the space between the stars, and the 21-centimeter wavelength is therefore a universal radiation that would be received anywhere. Any civilization that had reached our technological level would certainly be radio astronomers, and we can be certain they would have instruments equipped to receive the 21-centimeter wavelength even if they bothered with nothing else. Surely they would transmit messages over a wavelength they could themselves receive and one that they would be certain that all other civilizations would be tuned to.

In 1959, therefore, the American physicist Philip Morrison and the Italian physicist Giuseppe Cocconi (1914–) suggested that if signals from extraterrestrials were searched for, they should be searched for at 21-centimeter wavelengths.

That is the microwave wavelength, however, in which the background radiation is strongest and potentially the most obscuring—particularly in the region of the Milky Way. There is some feeling, therefore, that we ought to look somewhere else, perhaps at 42 centimeters or 10.5 centimeters, since doubling or halving the obvious choice is the simplest way of using 21 centimeters as the basis for the message without using that wavelength itself.

Another suggestion is to make use of hydroxyl, the 2-atom combination of hydrogen and oxygen, which, next to hydrogen itself, is the most widespread emitter of microwaves in interstellar space. Its microwave emission has a wavelength of 17 centimeters (6.7 inches).

Since hydrogen and hydroxyl together make water, the stretch of microwaves from 17 to 21 centimeters in wavelength is sometimes called the waterhole. The name is particularly apt, because the hope is that different civilizations will send and receive messages in this region as different species of animals come to drink at literal water-holes on Earth.

In 1960, the first real attempt was made to listen to the 21-centimeter wavelength in the sky in the hope of detecting messages from extraterrestrial civilization. It was carried through in the United States under the direction of Frank Drake, who called it Project Ozma. Ozma was Princess of Oz, the distant land in the sky of the well-known children’s adventure series. After all, the astronomers were trying to gain evidence of occupied lands even farther in the sky than Oz is.

The listening began at 4 A.M. on April 8, 1960, with absolutely no publicity, since the astronomers feared ridicule. It continued for a total of 150 hours through July, and the project then came to an end. The listeners were on the alert for anything with a very narrow range of wavelengths that seemed to flicker in a way that was neither quite regular nor quite random. They detected nothing of the sort.

Since Project Ozma, there have been six or eight other such programs, all at a level even more modest than the first, in the United States, in Canada, and in the Soviet Union. There have been no positive results, but the fact is that the search has been very brief and superficial so far.

Astronomers remain alive to the possibility of accidental discoveries, of course. When, in 1967, pulsars (very tiny, very dense, very rapidly rotating stars that were remnants of collapse following supernova explosions) were discovered, for just a short while the surprising detection of pulses of microwaves gave the astronomers concerned an eerie feeling that messages of intelligent origin were being received. They referred to it as the LGM (“little green men”) phenomenon. The pulses quickly proved far too regular to be carrying a message, however, and less dramatic explanations were found.

If the search for messages from extraterrestrial civilizations is to be carried through with some reasonable hope of success, however, far more time must be spent than was the case in Project Ozma; far more stars must be studied, far more elaborate equipment must be used. In short, a very expensive project must be set up.

WHERE?

In 1971, a NASA group under Bernard Oliver suggested what has come to be called Project Cyclops.

This would be a large array of radio telescopes,^* each 100 meters (109 yards) in diameter, and each adjusted for reception of microwaves in the waterhole region.

The array would consist of 1,026 such radio telescopes in rank and file, all of them steered in unison by a computerized electronic system. The entire array working together would be equivalent to a single radio telescope some 10 kilometers (6.2 miles) across.

The array would be capable of detecting something as weak as Earth’s inadvertent leakage of microwaves even from a distance of 100 light-years, while the deliberately emitted message beacon of another civilization could be detected at a distance of at least 1,000 light-years.

Earth’s surface may not be the best place for it. If it could be built in space, or, better yet, on the far side of the Moon, it would be insulated from most or all of the background of Earth’s own microwave noise.

Project Cyclops would not be easy to construct and certainly not cheap. Estimates are that the construction and maintenance of the array and the search itself would cost anywhere from $10 to $50 billion, even allowing for the fact that eventually the listening will be completely computerized and will not take much in the way of people-hours.

Anything that could be done to make the search simpler and quicker would be helpful, therefore. There might be places in the sky, for instance, where it would pay us to search first because they are more likely sources of messages than other places are.

Where might these places be?

First, the best place to search is in the neighborhood of some star where a planetary civilization with copious energy at its disposal might exist. (There might be, to be sure, signals being sent out by free-worlds or automatic probes that are closer to us than any star, but we have no way of knowing where such objects are and therefore no particular target to aim at.)

Second, the objective should be a nearby star rather than a distant star, since, all things being equal, the microwave beam will be more intense and easier to detect the closer the planetary system from which it starts.

Third, the objective should be a Sunlike star, since it is there we expect habitable planets might exist.

Fourth, the first objectives should be single stars, since, even though it seems that binary stars may still have habitable planets circling them, the chances are perhaps greater in the case of single stars.

As it happens, there are just seven Sunlike single stars within 2 dozen light-years of us, and they are:

None of these stars has a familiar name, for those that do are generally the brightest, which are too large and short lived to be suitable for civilizations.

Stars that are visible to the unaided eye, even if they are not outstandingly bright, are generally named for the constellation in which they are found. Sometimes they are listed in order of brightness, or position, by the use of Greek letters (alpha, beta, gamma, delta, epsilon, zeta, and so on) or by Arabic numerals.

The stars in the table above are from the constellations Eridanus (the River), Cetus (the Whale), Draco (the Dragon), Pavo (the Peacock), Hydrus (the Water Snake), and Tucana (the Toucan).

Of the seven stars listed in the table, three—Delta Pavonis, Beta Hydri, and Zeta Tucanae—are located so far south in the sky as to be invisible from the northern climes where astronomy is most advanced and where complex equipment exists in the greatest profusion. As for 82 Eridani, that is not too far south to be visible, but it is apt to be too near the horizon for complete comfort.

The three very best targets, then, are Epsilon Eridani, Tau Ceti, and Sigma Draconis. Project Ozma, at the suggestion of the Russian-American astronomer Otto Struve, concentrated on Epsilon Eridani and Tau Ceti.

Although these seven stars, and particularly the three northern stars, are the obvious targets for the first phase of the search, we should not quit if the results are negative. If there are seven prime targets within 23 light-years, there would be about 500,000 altogether within the 1,000-light-year reach of the Project Cyclops array.

Ideally, we should listen to all of them. In fact, before we really give up hope, we should scan the entire sky, just in case civilizations are present in the neighborhood of surprising stars—or just in case we get signals from probes or free-worlds that are fairly close to us without our being aware of it.

We should even search wavelength ranges outside the waterhole, just in case.

WHY?

Yet one must ask: Why ought humanity to engage in the task of monitoring space for signals from extraterrestrial civilizations? Why should we spend tens of billions of dollars when the chances are that we may find nothing at all?

After all, what if, despite all my reasoning in this book, there are no extraterrestrial civilizations?

—Or if there are, that there are none so close to us that we can detect their signals?

—Or if there are, that they are not signaling?

—Or if they are, that they are doing so in a way that will elude us altogether?

—Or if it doesn’t, that the signals we receive will be uninterpretable?

Any of these things is possible, so let us assume the worst and suppose that despite all our efforts, we end up with no recognizable signals at all from anywhere.

In that case, will we really have wasted much money?

Perhaps not. Suppose that the labor of building Project Cyclops and the task of searching the sky takes 20 years altogether and costs $100 billion. That is $5 billion a year in a world in which the various nations spend a total of $400 billion a year on armaments.

And whereas the money spent on armaments only stimulates hatred and fear and increases steadily the chance that the nations of the Earth will wipe out each other and, perhaps, all humanity, the search for extraterrestrial intelligence is something that would surely have a uniting effect on us all. The mere thought of other civilizations advanced beyond our own, of a Galaxy full of such civilizations, can’t help but emphasize the pettiness of our own quarrels and shame us into more serious attempts at cooperation. And if the failure of the search should cause us to suspect that we are, after all, the only civilization in the Galaxy, might that not increase the sense of the preciousness of our world and ourselves and make us more reluctant to risk it all in childish quarrels?

But will the money be wasted at all if we end up with nothing?

In the first place, the very attempt to construct the equipment for Project Cyclops will succeed in teaching us a great deal about radiotelescopy and will undoubtedly advance the state of the art greatly even before so much as a single observation of the heavens is made.

Secondly, it is impossible to search the heavens with new expertise, new delicacy, new persistence, new power, and fail to discover a great many new things about the Universe that have nothing to do with advanced civilizations. Even if we fail to detect signals, we will not return from the task empty-handed.

We can’t say what discoveries we will make, or in what direction they will enlighten us, or just how they may prove useful to us, but humanity has (at its best moments) always valued knowledge for its own sake. The ability to do that is one of the ways in which a more intelligent species would be differentiated from a less intelligent one; and an advancing culture is differentiated from a decaying one.

Nor need we fear that in the end knowledge will have to be valued for its own sake only. Knowledge, wisely used, has always been helpful to humanity in the past; and there is every hope it will continue to be helpful in the future.

But suppose we do find a signal of some sort and decide that it must be of intelligent origin. Will that be of great value to us?

It may be that it won’t be a beacon at all; that no one is trying to attract our attention or to tell us anything. It may be the inadvertent overflow of technology, just a jumble of everyday activity, like the ball of microwaves that is now steadily expanding from the Earth in every direction.

That in itself—the mere recognition of a signal as representing the existence of a far-off civilization, even one from which we can extract no information at all—is quite enough, in some ways.

Think of the psychological significance right there. It means that somewhere else a civilization exists,^* which, judging from the mere strength of its signals, might just be advanced beyond our own. That alone gives us the heartening news that at least one group of intelligent beings has reached our level of technology and has succeeded in not destroying itself, but has instead survived and advanced onward to greater heights. And if they have done so, may we not do so as well?

If this thought helps keep us from despair during humanity’s mountainous tasks of solving the problems that lie immediately ahead of us, that alone may help move us toward the solution. It might even, perhaps, provide the crucial feather’s weight that may swing the balance toward survival and away from destruction.

Nor can it be possible that we will get no information other than the mere existence of the signal. Even if there is no intelligent message in the signal, or none that we can interpret, the characteristics of the signal could tell us the rate at which the signal-sending planet revolves about its star and rotates about its axis, together with perhaps other physical characteristics that could be of great interest and use to astronomers.

And suppose we recognize that there is useful material in the message, yet remain at a total loss to determine what that useful material might mean.

Is the message then useless? Of course not. In the first place, it presents us with an interesting challenge, a fascinating game in itself. Without coming to any conclusion as to specific items of information, we might reach certain generalizations concerning alien psychologies—and that, too, is knowledge.

Besides, even the tiniest breaks in the code could be of interest. Suppose, for the sake of argument, that from the message we get the hint of a relationship that, if true, might give us a new insight into some aspect of physics—it might even seem a trivial insight. Yet scientific advances do not exist in a vacuum. That one insight might stimulate other thoughts and, in the end, greatly accelerate the natural process by which our scientific knowledge advances.

And if we do come to some detailed understanding of the message, we might learn enough to be able to deduce whether the civilization sending it is peaceful or not.

If it is dangerous and warlike (a very slim chance, in my opinion), then the knowledge we will have gained will encourage us to keep quiet, make no reply, do our best to shield as far as possible any leakage into outer space of anything that will give a hint of our presence. Perhaps the knowledge we gain will give us some insight into how best to defend ourselves if the worst comes to the worst.

If, on the other hand, we decide that the messages are coming from a peaceful and benign civilization, or from one that cannot reach us whatever its attitude, then we might decide to answer, using the code we have learned.

To be sure, the civilization may be so far away from us that, thanks to the speed-of-light limit, we cannot expect an answer for, say, a century. There is, however, no great problem in waiting. We can go about our own business while we wait, so we lose nothing.

The advanced civilization at the other end, on receiving our answer and knowing that someone is listening, may perhaps at once begin to transmit in earnest. Though we wait a century for it, we would find ourselves thereafter getting a cram-course in all aspects of the alien civilization.

There is no way we can predict how useful such information will prove to be, but surely it cannot be useless.

In fact, if we move to the romantic extreme of supposing that the speed-of-light limit can be beaten and that there is a peaceful and benign Federation of Galactic Civilizations, our successful interpretation of the message and our courageous answer may amount to our ticket of entrance.

Who knows?

Even disregarding the vast curiosity that has always driven humanity, and the intense interest we all must have in so overwhelming a question as to whether or not there are other civilizations in the Universe in addition to our own, it does seem to me that no matter what we do in attempting to answer that question, we will succeed in profiting and in helping ourselves.

Therefore, for the sake of all of us, let’s abandon our useless, endless, suicidal bickering and unite behind the real task that awaits us—to survive—to learn—to expand—to enter into a new level of knowledge.

Let us strive to inherit the Universe that is waiting for us; doing so alone, if we must, or in company with others—if they are there.